1. Field of Invention
This invention relates to high power, infra-red lasers, more specifically, efficient laser diode excited solid state pumped molecular gas lasers and amplifiers.
2. Description of Prior Art
Mid-infrared vibrational-rotational transition lasers are well known. They are potentially very important as they have high power/high energy capability within critical atmospheric windows in the mid-wave infrared (MWIR) and long wave infrared (LWIR) spectral regions. These systems are typically energized using chemical interactions or electrical discharges. Chemically pumped systems are undesirable from the standpoint of the reactive precursors required, plus toxic exhaust product handling or release considerations. Electrical discharge pumped Carbon Monoxide (CO) lasers are less than successful as they generally emit at wavelengths greater than approximately 5.6 μm which is above the approximately 4.6 μm to approximately 5.4 μm atmospheric transmission window. There is a clear need to replace chemical or electrical discharge excitation of these devices.
U.S. Pat. No. 7,145,931 (Diode Pumped Alkali-Molecular Lasers and Amplifiers) describes optically-pumped mid-infrared vibrational-rotational transition gas lasers and amplifiers with improved efficiency and practicality, to with, inventive lasers and amplifier devices include: laser active media comprising a mixture of alkali vapor, selected hetero-nuclear molecular gas, and one or more buffer gases; conventional semiconductor laser diode pump sources with nanometer scale spectral bandwidths; and preferential laser emission in rovibrational transitions among relatively low-lying vibrational levels.
This is a laser diode pumped resonant transfer approach and is deficient for a number of reasons. It is deficient in that it is not configured for direct and selected excitation of higher vibrational levels as this is a resonant transfer approach, as distinct from the concept proposed herein where multiple principle and excited state overtone pumps are applied [FIG. 1, A] in an intentional manner. This would have direct consequences in terms of the spectral diversity produced by the system which is directly related to the number of fundamental cascade (v→v−1 transitions—[FIG. 1, B]. v is the vibrational level quantum number designator) elements present in lasing. Furthermore, direct pump spectral location has an influence on cascade formed; that is, it exerts a degree of selection on content of the spectral output. The patent cited is deficient in that any dissociated halogen components derived from vv exchange up pumping will scavenge alkali atomic vapor components via principally three body interactions eroding donor/acceptor gas mix balance. Recombined diatomic homonuclear halogens will react with alkali atomic vapor components as M+X2→X+MX (M is alkali vapor component—X is halogen). Alkali atomic vapor components have the ability to enter into M+HX→H+MX, M*+HX→H+MX and M*+HX*→H+MX (HX is acceptor component of U.S. Pat. No. 7,145,931—M is atomic vapor component—X is atomic halogen—* superscript denotes excitation, H is nominal, it may equally well be D) exchange interactions at operational temperatures concerned also eroding donor/acceptor gas mix balance. The latter issues clearly do not lend themselves to implementation of a closed cycle gas operating system free of precursor consumption or product handling. In addition, the system has to be conditioned for alkali vapor generation—typically in the range of 300K to 500K. It is deficient in that a laser diode pumped system of this nature lacks the ability to function in a pulsed significantly high power/high energy mode of operation as the system lacks lasing medium energy storage capability. This is also distinct from the invention presented here which admits a laser diode excited bulk pulsed Tm solid state driver and has the capability to store and deliver high energy optical pulses with approximate extractions of 4 kJ/liter, plus closed cycle operation of gas component of system which, when in full cascade, merely functions as a throughput device with wavelength shift and output proportional to input in full cascade.
Additionally it has been asserted in the above cited patent (U.S. Pat. No. 7,145,931) that direct optical pumping of molecular transitions necessarily results in reduced performance from the pump source system because of the need to narrow the line width of these sources to match the line width of the molecular transitions concerned. Typically, for the gas system process presented herein, a rotational-vibrational line width will be in the range of approximately 0.5 GHz to approximately 2 GHz. This is equivalent to, for the pump concerned, approximately 0.007 nm to approximately 0.03 nm. Greater line widths are not excluded as they are a function of selected operating gas conditions which are not confined or restricted in any way by the identified typical line widths. However in general, such interaction line widths do traditionally diminish solid state laser performance courtesy of the limitations of cross relaxation. However, the cited patent's assertion is deficient as this problem is amenable to the following mitigating features particular to this invention: (a) The specific full to ground sustained lasing cascade process, in a suitable gas optically pumped, can only arise for pump pulse durations significantly greater than several to many times the rotational manifold thermalization time constant of the pumped gas, plus the duration associated with formative lasing onset supported level to level population redistribution supportive of creation of the conditions for sustainment of the full lasing cascade to ground. Typically this implies that optical pump durations greater than several hundred nanoseconds are required. Thus short pulse interactions are not suitable and not under consideration and thus the rapid cross relaxation in highly Tm doped YAG (Thulium doped Yttrium Aluminum Garnet), for example, in conjunction with its long excited state lifetime of approximately 10 ms will allow for efficient narrow band extraction. Similarly cross relaxation in Tm:glass, as in fibers, is also rapid at high dopant levels. Solid state Tm doped pulsed systems tend to produce, or can be configured to produce, pulse lengths in the range desired which is greater than several hundred nanoseconds to microseconds; a case with Ceramic Tm:YAG has recently been demonstrated, the implication of which is that arbitrarily configured and scaled highly doped amplifier structures can be fabricated. (b) In the case of continuous wave (CW) fiber pumps, the medium is characterized by an inhomogeneously broadened gain distribution. It is possible to amplify several spectrally separate wavelength components in such a medium provided that they are spectrally separated by the homogenous line width of the molecular subgroup (sometimes described as the mode repulsion range)—thus enabling useful enhancement of the effective interaction bandwidth and thus enhancement of solid state system performance. This fact has natural synergism with the principle plus multi excited state overtone or multi principle overtone (differing rotational vibrational transitions) gas pump feature of the invention presented here. To a degree this behavior would also manifest for pulsed Tm:YAG, or YAP, YSGG or any other suitable solid state host, as system is quasi 4 level with different wavelengths between differing Stark components of the energy level structure and in response, inhomogeneously broadened.
This approach is also deficient, in regards of output spectral range accessible and thus for specific applications, in terms of the fact that resonant transfer systems in general (and as stated in the abstract (Krupke: 7,145,931)) offer preferential laser emission in ro-vibrational transitions among relatively low-lying vibrational levels. This as indicated will limit the spectral diversity achievable by pumping of higher vibrational levels which is directly addressable and may be intentionally tailored within the context of the invention presented here.
The article, CW Optical Resonance Transfer Lasers. [J. H. S. Wang et al, Journal de Physique, Colloque C9, supplement au no 11, Tome 41, 1980, C9-463] describes wavelength-agile, single and multiline laser radiation that has been obtained from a subsonic gas flow system which is optically pumped with a multiline chemical laser. This optical resonance transfer laser (ORTL) concept was first demonstrated on the 10.6 μm DF/CO2 system in 1976. Since then, several infrared (IR) laser pumped molecular lasers have been demonstrated. The pump laser is either a CW (Continuous Wave) HF or DF chemical laser. Two classes of ORTL have been developed: inter- and intramolecular ORTLs. The demonstrated intermolecular systems include: 10.6 μm DF/CO2, 10.8 μm DF/N2O, 4.1 μm DF/HBr, 3.8 μm HF/DF and 3.85 μm HF/HCN. The intramolecular ORTLs include 2.9 μm HF/HF and 3.9 μm DF/DF. Demonstration experiments and the kinetics of ORTL systems will be described.
This was a fundamental cascade optically pumped followed by a fundamental cascade lasing response system. The approach of this article is deficient as there is no practical value to optically pumping HF with another HF laser, or DF with DF. That is, there is no useful wavelength shift induced between pump input and output. The approach of this article is also deficient in that the pump chemical HF, or DF, laser system utilized is impractical. Chemical lasers although in general efficient require exhaust product plus precursor fuel and oxidizer handling. If purely cold reaction discharge initiated by dissociation of halogen donor, then are typically inefficient plus still require product gas handling and high voltage.
The article, CW Optically Resonance Pumped Transfer Laser in DF-CO2System [J. H. S. Wang et al, Applied Physics Letters, 31(1), 1977, 35-37] describes an optically pumped CW 10.6 μm DF/CO2 transfer laser that has been demonstrated. This has been accomplished by exciting a 3 cm×0.3 cm transfer laser medium, consisting of a 1:19:80 DF/CO2/He flowing gas mixture at 22 Torr and room temperature, with a 70 Watt multiline chemical DF laser. In the preferred ‘intracavity’ configuration where the transfer-laser medium was located in between the DF-laser resonator mirrors, 1.5 W of 10.6 μm power was out coupled. This power corresponds to a photon conversion efficiency of available DF-pump flux to out coupled transfer-laser flux of 6%. Analysis predicts that multiline DF laser-to-single-line CO2 photon conversion efficiencies exceeding 90% should be attainable in an optimized apparatus configuration.
This approach is deficient in that the pump laser is impractical. Chemical lasers although in general efficient require exhaust product plus precursor fuel and oxidizer handling. If purely cold reaction discharge initiated by dissociation of halogen donor, then are typically inefficient plus still require product gas handling and high voltage.
The article CW HF/HCN and HF/DF Optical Resonance Transfer Lasers [J. Finzi et al, IEEE Journal of Quantum Electronics, QE-16(9), 1980, 912-914] describes that CW laser oscillation has been observed in HCN at 3.85-3.9 μm and in DF at 3.8-4.0 μm. Mixtures of HF/HCN/He and HF/DF/He were irradiated by a CW multiline HF chemical laser. Vibrational excitation of HF by resonance absorption, followed by rapid v-v energy transfer to HCN or DF, produced a population inversion. The HCN gain was estimated to be between 0.08 and 0.17 percent/cm. The DF gain was greater than 0.17 percent/cm and 25 mW of power were out coupled.
This approach is deficient in that the pump laser is impractical. Chemical lasers although in general efficient require exhaust product plus precursor fuel and oxidizer handling. If purely cold reaction discharge initiated by dissociation of halogen donor, then are typically inefficient plus still require product gas handling and high voltage.